The present invention relates to Ziegler-Natta catalyst compositions for use in the polymerization of olefins having improved control over polymer properties through the use of carefully chosen mixtures of selectivity control agents. Ziegler-Natta catalyst compositions are well known in the art. Typically, these compositions include a transition metal polymerization catalyst, especially a titanium containing compound; a co-catalyst, usually an organoaluminum compound; and a selectivity control agent (SCA), usually an organosilicon compound or mixture thereof. Examples of such Ziegler-Natta catalyst compositions are shown in U.S. Pat. No. 5,247,032; U.S. Pat. No. 5,247,031; U.S. Pat. No. 5,229,342; U.S. Pat. No. 5,153,158; U.S. Pat. No. 5,151,399; U.S. Pat. No. 5,146,028; U.S. Pat. No. 5,106,806; U.S. Pat. No. 5,082,907; U.S. Pat. No. 5,077,357; U.S. Pat. No. 5,066,738; U.S. Pat. No. 5,066,737; U.S. Pat. No. 5,034,361; U.S. Pat. No. 5,028,671; U.S. Pat. No. 4,990,479; U.S. Pat. No. 4,927,797; U.S. Pat. No. 4,829,037; U.S. Pat. No. 4,816,433; U.S. Pat. No. 4,728,705; U.S. Pat. No. 4,562,173; U.S. Pat. No. 4,548,915; U.S. Pat. No. 4,547,476; U.S. Pat. No. 4,540,679; U.S. Pat. No. 4,472,521; U.S. Pat. No. 4,460,701; U.S. Pat. No. 4,442,276; U.S. Pat. No. 4,439,540; U.S. Pat. No. 4,330,649; U.S. Pat. No. 4,294,721; U.S. Pat. No. 4,220,554; U.S. Pat. No. 4,115,319; and U.S. Pat. No. 4,107,413.
Catalyst compositions designed primarily for the polymerization of propylene or mixtures of propylene and ethylene generally include a selectivity control agent in order to affect polymer properties, especially tacticity or stereoregularity of the polymer backbone. As one indication of the level of tacticity, especially the isotacticity of polypropylene, the quantity of such polymer that is soluble in xylene or similar liquid that is a non-solvent for the tactic polymer is often used. This is referred to as the xylene solubles content of the polymer, or XS. In addition to tacticity control, molecular weight distribution (MWD), melt flow (MF), oligomer content, and other properties of the resulting polymer are often affected, sometimes adversely, by use of SCA's. For example, many SCA's that give desirable control over XS, give polymers of such high molecular weight so as to detrimentally affect the MF of the polymer. Increasing amounts of a chain transfer agent, such as hydrogen, may be employed in order to provide desirable MF properties in the resulting polymer. Alternatively, a post reactor treatment such as use of peroxides, may be used to reduce polymer chain length. These techniques however, result in increased cost, elevated oligomer content, and can introduce undesired volatile components into the product. Accordingly, SCA's having less adverse affect on MF properties or reduced hydrogen sensitivity, while retaining good XS properties are still desired.
Known SCA's include individual tetraalkoxysilanes, such as tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS). Although these SCA's are capable of producing high melt flow polymers at reduced hydrogen levels, the activity of most Ziegler-Natta procatalysts is generally adversely affected by their use. In addition, isotacticity control, also referred to as catalyst selectivity, using TEOS as the sole SCA is generally poor, leading to polypropylene products having high xylene solubles content, that is XS values greater than 3.0 percent or even greater than 5.0 percent.
Use of mixtures of SCA's in order to adjust polymer properties is also known. Examples of prior art disclosures of catalyst compositions making use of mixed SCA's include: U.S. Pat. No. 6,337,377; U.S. Pat. No. 6,303,698; U.S. Pat. No. 6,184,328; U.S. Pat. No. 6,147,024; U.S. Pat. No. 6,133,385; U.S. Pat. No. 6,127,303; U.S. Pat. No. 6,111,039; U.S. Pat. No. 6,096,844; U.S. Pat. No. 6,087,459; U.S. Pat. No. 6,066,702; U.S. Pat. No. 6,001,765; U.S. Pat. No. 5,869,418; U.S. Pat. No. 5,849,654; U.S. Pat. No. 5,844,046; U.S. Pat. No. 5,652,303; U.S. Pat. No. 5,414,063; U.S. Pat. No. 5,192,732; U.S. Pat. No. 5,100,981; WO 99/58585; WO 99/20663; and WO95/21203.
The foregoing mixtures of SCA's generally include various combinations of alkyl- or cycloalkyl-substituted siloxanes, such as dicyclopentyldimethoxysilane (DCPDMS), propyltriethoxysilane (PTES), methylcyclohexyldimethoxysilane (MCHDMS); and mixtures of the foregoing mixed silane compounds, or mixtures thereof with individual tetraalkoxysilane compounds. Generally these combinations give polypropylene polymers having better XS values, but require the use of large hydrogen concentrations or peroxide post treatment to obtain desired melt flow values.
Despite the advances occasioned by the use of the foregoing SCA mixtures, there remains a need in the art to provide a Ziegler-Natta catalyst composition for the polymerization of olefins comprising the combination of a Ziegler-Natta catalyst with a mixture of selectivity control agents that results in good selectivity control, evidenced by production of polymers having reduced or acceptable xylene soluble content (XS), especially XS≦3.0 weight percent, in combination with good hydrogen sensitivity and good catalyst activity. Especially desired is the ability to produce moderate or high melt flow polymers at relatively low H2 concentrations with good catalyst efficiency.